The purpose of this study is to establish the mechanism(s) of progression of communicating syringomyelia. Communicating syringomyelia usually accompanies abnormalities at the craniocervical junction. Measurement of intraventricular pressure, intrathecal pressure, and intrasyrinx pressure provide data which elucidate the hydrodynamic mechanism(s) of progression of syringomyelia. Radiographic testing, including MRI flow studies, ultrasonography, and Imatron CT, is demonstrating how pathologic anatomy alters normal CSF. The effect of posterior fossa craniectomy, upper cervical laminectomy, and duraplasty on CSF flow, syrinx size, and neurologic function is being evaluated. Five patients have been treated. No patient had communication between the 4th ventricle and the syrinx. Ultrasonographic measurements demonstrated cord and syrinx constriction during systole. Despite obstruction of CSF pathways at the foramen magnum, phase and cine-MRI demonstrated pulsatile syrinx and cervical subarachnoid CSF flow. CSF pressure measurements confirmed the transmission of intracranial pressure to the cervical subarachnoid space and the syrinx. Because intracranial pressure is transmitted despite obstruction of the subarachnoid space at the foramen magnum, we conclude that the cerebellar tonsils and the brainstem act on a partially enclosed spinal subarachnoid space to generating cervical subarachnoid CSF pressure waves. These waves compress the spinal cord from without, not from within, as has previously been considered to occur, to propel the syrinx fluid downward with each heart beat. Syrinx progression occurs as a consequence. Craniocervical decompression and duraplasty improved CSF at the foramen magnum in all patients. The syringes decreased in size following surgery. The pressure measurements have been performed without complication. We plan to proceed with an additional 5 patients.